1. Field of the Invention
The invention generally relates to receivers for television signals and more particularly to receivers including teletext decoders for use in a teletext transmission system.
2. Description of the Prior Art
As is generally known, in a teletext transmission system, a number of pages is transmitted from a transmitter to the receiver in a predetermined cyclic sequence. Such a page comprises a plurality of lines and each line comprises a plurality of alphanumerical characters. A character code is assigned to each of these characters and all character codes are transmitted in those (or a number of those) television lines which are not used for the transmission of video signals. These television lines are usually referred to as data lines.
Nowadays the teletext transmission system is based on the standard known as "World System Teletext", abbreviates WST. According to this standard each page has 24 lines and each line comprises 40 characters. Furthermore each data line comprises, inter alia, a line number (in a binary form) and the 40 character codes of the 40 characters of that line.
A receiver which is suitable for use in such a teletext transmission system includes a teletext decoder enabling a user to select a predetermined page for display on a screen. As is indicated in, for example, Reference 1, a teletext decoder comprises, inter alia, a video input circuit (VIP) which receives the received television signal and converts it into a serial data flow. This flow is subsequently applied to an acquisition circuit which selects those data which are required for building up the page desired by the user. The 40 character codes of each teletext line are stored in a page memory which at a given moment thus comprises all character codes of the desired page. These character codes are subsequently applied one after the other and line by line to a character generator which supplies such output signals that the said characters become visible when signals are applied to a display.
For the purpose of display each character is considered as a matrix of m.sub.1 .times.m.sub.2 picture elements which are displayed row by row on the screen. Each picture element corresponds to a line section having a predetermined length (measured with respect to time); for example, q.mu.sec. Since each line of a page comprises 40 characters and each character has a width of m.sub.1 q.mu.sec, each line has a length of 40 m.sub.1 .mu.sec. In practice a length of approximately 36 to 44 .mu.sec appears to be a good choice. In the teletext decoder described in Reference 1 line length of 40 .mu.sec and a character width of 1 .mu.sec at m.sub.1 =6 have been chosen.
The central part of the character generator is constituted by a memory which is sub-divided into a number of submemories, for example, one for each character. Each sub-memory then comprises m.sub.1 .times.m.sub.2 memory locations each corresponding to a picture element and the contents of each memory location define whether the relevant picture element must be displayed in the so-called foreground colour or in the so-called background colour. The contents of such a code memory location will be referred to as character picture element code. This memory is each time addressed by a character code and a row code. The character code selects the sub-memory and the row code selects the row of m.sub.1 memory elements whose contents are desired. The memory thus supplies groups of m.sub. simultaneously occurring character picture element codes which are applied to a converter circuit. This converter circuit usually includes a buffer circuit for temporarily storing the m.sub.1 substantially presented character picture element codes. It is controlled by display clock pulses occurring at a given rate and being supplied by a generator circuit. It also supplies the m.sub.1 character picture element codes, which are stored in the buffer circuit, one after the other and at a rate of the display clock pulses. The serial character picture element codes thus obtained are applied to a display control circuit converting each character picture element code into an R, a G and a B signal value for the relevant picture element, which signal values are applied to the display device (for example, display tube).
The frequency f.sub.d at which the display clock pulses occur directly determines the length of a picture element and hence the character width. In the above-mentioned case in which m.sub.1 =6 and in which a character width of 1 .mu.sec is chosen, this means that f.sub.d =6 MHz. A change in the rate of the display clock pulses involves a change in the length of a line of the page to be displayed (now 40 .mu.sec). In practice a small deviation of, for example, not more than 5% appears to be acceptable. For generating the display clock pulses the generator circuit receives reference clock pulses. In the decoder circuit described in Reference 1 these reference clock pulses are also supplied at a rate of 6 MHz, more specifically by an oscillator specially provided for this purpose.